Soluble coffee products are produced by extracting soluble coffee solids from ground roasted coffee with hot water, e.g., 150.degree. C. to 180.degree. C. The resulting extract is concentrated and dried, such as by spray drying or freeze drying, to provide soluble coffee powder. Extraction efficiency in such a manufacturing operation is influenced by a number of factors including grind of coffee, temperature of extraction water, prewetting of the ground coffee, design of extraction equipment and flow rate of extraction liquid through ground coffee particles.
Commercial production of soluble coffee products commonly is carried out in a counter-current cell extraction system in which hot water is introduced into one end of a plurality of extraction columns operated in series, with the water passing through one column to another and successively contacts progressively fresher coffee grounds. The last column contains the freshest (or least extracted) coffee grounds. Commercial cell extraction systems typically have from about 4 to 10 or more extraction columns, each one filled with a bed of roast and ground coffee. In such counter-current extraction systems, water entering the system at one end becomes progressively more concentrated with dissolved coffee solids as it contacts progressively fresher coffee grounds in successive columns. In commercial operations, roast and ground coffee, is filled into an extraction column and prewetted with extraction liquid, usually water or coffee extract. Prewetting of roast and ground coffee in an extraction system is known to provide a number of advantages, such as improving cup flavor and improving extraction efficiency. However, uniform prewetting of coffee particles is difficult to achieve using procedures which have been available heretofore. Non-uniform prewetting of the coffee particles causes large pressure drops across the extraction columns which may result in increased extraction cycle time and decreased yield. Also, coffee gases evolved upon contacting the coffee particles with water impede complete wetting of the coffee particles.
During coffee extraction in the system, coffee gases are also evolved upon contact of the ground coffee with extraction liquid, with the evolved coffee gases being trapped within the bulk mass of coffee grounds in the extraction column. In general, the trapped coffee gases exist in a foamy state and result in a general reduction in the efficiency of the coffee extraction operation. Thus, the presence of the foamy gases in the mass of coffee grounds in a column impedes the extraction process by preventing good contact between the extraction liquid and coffee grounds, increases flow resistance of the liquid through the coffee grounds, and significantly reduces the mass transfer coefficient between extraction liquid and coffee grounds. In addition, the trapped coffee gases cause the bulk coffee mass to float on the extraction liquid thereby impeding complete wetting of the coffee particles and preventing fully wetted particles from settling out of the coffee mass.
Due in large part to the inability to achieve uniform prewetting of coffee particles and to problems resulting from the generation of foamy coffee gases in the bulk mass of coffee grounds during extraction, attempts to develop procedures for the continuous countercurrent liquid phase extraction of ground roasted coffee have not been successful to date on a commercial scale.